Dual tone telegraphy system



May 31, 1938. w. G. H. FlNcH DUAL TONE TELEGRAPHY SYSTEM Filed Sept. I5, 1936 2 Sheets-Sheet 1 ENPJTL NNIL/ INVENTOR ATTORNEY May 3l, 1938. w. G. H. FlNcH DUAL TONE TELEGRAPHY SYSTEM Filed Sept. 3, 1956 2 Sheets-Shea*I 2 INVENTOR will am/g Mpg-inch ATTORNEY Patented May 31, 1938 UNITED STATES PATENT Vori-1ct Y DUAL TONE TELEGBAPHY SYSTM 5 Claims.

5 narrow frequency spread.

Continuous wave telegraph systems of the prior art comprised a single carrier frequency which was either modulated by the telegraphy signals or interrupted in accordance with the signals.

0 One prior art system extensively used is to transmit the carrier frequency wave during the marking periods of the signals and .to key-off or prevent transmission of the carrier wave during the spacing periods. The received signals are heterodyned with a local oscillator to produce an audio frequency signal of predetermined frequency at the receiver corresponding .to the marking impulses of the transmitter. One disadvantage of this system is that the marking impulse is positively actuated whereas the spacing impulse is necessarily bias actuated. Accordingly, the word rate is not as great as in a system employing positive actuation of both marking and spacing impulses. Another disadvantage is that the separated carrier signal impulse trains excite the receiver tuned circuits resulting in a reception which is not as selective as an uninterrupted carrier wave transmission.

A radio telegraphy system which overcomes these disadvantages utilizes la radio frequency carrier which is modulated by two audio frequency tones separated suiiiciently so as to be filtered into respective marking and spacing relay members. Each of two frequencies, for example 300 cycles and 800 cycles, are caused to alternately modulate the radio frequency carrier in accordance with telegraphic impulses to be transmitted. The carrier is accordingly continuously transmitted and the selectivity materially increased. The individual marking and spacing audio frequency tone impulses provide positive actuation at the receiver. However, modern telegraphic radio channels are crowded and accord-- ingly, the permissible frequency spread assignable I employ two carrier frequency oscillators sepa rated by a small frequency difference. These oscillators are crystal controlled in order to insure accurate control of the frequency of the carrier waves. The .two frequencies correspond to the marking and spacing impulses of the coded telegraphy signals to be transmitted. Transmission of the closely spaced carrier frequencies is alternately effected in accordance with the marking and spacing transmission ,impulses to provide a substantially continuous transmission current which may be readily received with high selectivity. 'I'he amplitudes of the carrier waves in my preferred system are equal. so that an oscillographic view of the signal transmission would appear as a substantially continuous wave front with a frequency change of a. small predetermined amount between marking and spacing conditions.

'Ihe receiver of my novel system comprises a crystal controlled heterodyne oscillator which beats with the received signals to produce two audio frequency tones. 'Ihe frequency difference between the .two audio frequency tones is equal to the frequency difference between the two radio frequency carriers. By employing selective ltering, the two tones are individually impressed upon the respective marking and spacing members of the receiver relay to effect a positive marking and spacing actuation.

My present invention combines all the advantages of the continuous wave telegraphy and two tone modulation telegraphy systems. 'Ihe transmitted signal has a substantially constant intensity and may be very selectively received. I have performed very high speed operation over greater distances than with 'other radio telegraphic systems due to the sustained character of the energy of transmission, the sharp selectivity features of the reception and the positive actuation of the marking-spacing relay.

It is accordingly an object of my present invention to provide a novel high-speed radio telegraphy system.

Another object of my invention is to provide a novel high speed radio telegraphy system operating on a very narrow frequency 'spr/cad.

Afurther object of my invention is to provide novel circuit arrangements for transmitting and receiving radio telegraphic signals.

These and other objects of my invention will become apparent in the following description taken in connection with the drawings, in which:

Figure 1 is a circuit diagram of a preferred condenser 2 I.

transmitter for carrying out my present invention; and

Figure 2 is a circuit diagram of a preferred receiver for carrying out my present invention.

A preferred circuit arrangement for the transmitter is schematically illustrated in Figure l. Two separate radio frequency oscillators A and B are used to generate the marking and spacing radio frequency carrier waves. These oscillators are crystal controlled in my preferred system to insure accurate transmission and reception and to insure the predetermined diil'erence frequency between the transmitted carrier waves and to insure that .the transmission is carried out within assigned frequency limits. 'I'he output of the oscillators is amplified by radio frequency amplifier stages I0 and Il and by the buffer stage amplifiers I2 and I3 respectively. The output of the buffer amplifiers I2 and I3 is introduced to respective amplier stages Il and I6. i

The output connections I6 and II of the amplifier trains for oscillators A and B respectively, are connected to a common radio frequency ampliiler and transmitter I6. In Figure 1, there is illustrated a coupling between output points I6 and I`I to the transmitter I8 through respective resistances I9 and 26 and a common coupling The output of radio frequency transmitter I6 is radiated into space by antenna 22, in a manner well known in the art. The signal radiated by antenna 212 is two closely spaced radio frequency carriers alternately transmitted in a substantially continuous train.

The signal voltages at the outputs I6 and I1 of the oscillator-amplifier trains are arranged to be a maximum or zero in accordance with the "marking and "spacing signals to be transmitted. Should the frequency of radio frequency oscillator A be assigned for transmitting the marking signal impulses, then the output at mum during each marking" impulse and zero during the transmission of the "spacing impulses. Accordingly, the output at point I 'I of the B radio frequency oscillator train assigned to spacing impulses would be a maximum during the spacing impulse conditions and zero during the marking impulse conditions.

In accordance with my present invention, the carrier frequency output of each amplifier train is transmitted for the duration of signal impulse periods, alternately and corresponding to the marking and spacing signals of telegraphic transmission.

'I'his invention is applicable for the transmission of dot-dash telegraphic signals such as result from keying operations but has particular advantageous operation in the transmission of the marking and spacing coded signal combinations resulting from well known automatic printing telegraphic communication operating on, for example, a ve-unit Baudot code system. I prefer to employ any form of block signalling generator in combination with my novel transmitting and receiving system. At 23 I have schematically illustrated a signal selector mechanism which, it is to be understood, refers to any preferred form of established code selecting or generating device. The preferred tape and mechanical feeler combination operating with a rotary distributor is` one form of the selector 23 whereby high speed operation is feasible. However, any automatic or manual keying device may be utilized to generate successive'slgnal impulses at the outputs 24 and 25 of selector 23 corresponding to anaal? alternate marking and spacing signal conditions.

In order to limit the frequency spread transmitted and further stabilize the circuits, keying noise Suppressors 26 and 21 are connected to the outputs 24 and 26. Suppressors 26 and 2I are essentially nltercircuits designed to by-pass the harmonic frequencies of keying or signal impulse generation. 'I'he outputs of suppressors 26 and 2'I are connected respectively to marking and spacing solenoids 26 and 23 of relay 36. The tongue 3| of the signal relay is accordingly attracted by solenoids 26 and 26 to correspondingly connect to contacts 32 and 33 of the relay in accordance with the marking or spaclng" impulse conditions to be transmitted.

In accordance with my present invention. one radio frequency carrier is transmitted at full amplitude for the duration of one signal condition to be transmitted and another radio frequency carrier is transmitted at full amplitude v mined. For commercial application, the difference frequency may vconveniently be made seventy cycles to confine the frequency band transmitted to within narrow limits to fall within communication traillc assignments for narrow band widths. The substantially sustained character vof the transmitted signals permit selective reception at relatively greater distances for a given antenna power output since both the marking and spacing signal impulses .are transmitted at maximum carrier amplitudes.

'I'he substantially sustained character of the transmitted signals produce, in effect, a continuous carrier wave 'signalling condition. The advantage of such signalling condition resides in ythat a very selectively tuned receiver may be used to receive the signals and sharply reject 'adjacent signal bands in the crowded communication channel. The substantially continuous wave signalling permits the use of a receiver with sharply tuned circuits of relatively low decrement factor. Interrupted continuous wave sig` nailing is disadvantageous in that less sharply selective receiving circuits can be used The substantially continuous wave signalling produced by my present invention causes positive actuation of the receiver translating relay in accordance with the two tones conesponding to the two closely separated independent radio frequency carrier currents. By employing stable circuit arrangements permitting a low frequency difference, a very narrow channel provided for telegraphic communication may be used with my system. I shall hereinafter refer to the narrow frequency separation, preferably pf the order of cycles, as a low frequency difference for the two independent radio frequency carriers.

The keying of the A and B carrier frequency waves to alternately impress them upon the transmitter I8 in correspondence with the marking and spacing signals to be transmitted, may be effected in different ways. I prefer to accomplish the keying of the characters electronically for insuring rapid and inertialess operation in switching the carrier waves to and from the anaal? transmitter. A preferred manner for alternately keying the oscillator A and B outputs is accomplished by changing the bias conditions of the screen grid electrodes 34 and 35 of the respective pentode amplifier stages I4 and I5 as illustrated in Figure l. 'I'he other components of the amplifier stages I4 and I5 are connected in a contentional manner. Screen grid 34 is connected to relay contact 32 by connection lead 33; screen grid 35 is connected to relay contact 33 by connection lead 31. A source of potential 33 is connected between relay tongue 3i and ground. The positive terminal of the battery 33 is connected to the relay tongue 3| to provide suitable operating potential for the screen grid electrodes 34 or 35 in accordance with the relay contact 32 or 33 being contacted.

In the illustrated embodiment, relay tongue 3| is shown contacting relay contact 32 to impress a positive bias upon screen grid 34 to permit normal functioning of the amplifier stage I4. The amplified carrier frequency current from oscillator A is accordingly further amplified by stage I4 and directly impressed on the R. F.

transmitter i8 to be radiated by antenna 22. During the normal functioning of amplifier stage I4 due to the positive screen grid bias thereof, the amplifier stage I5 is choked since the potential from battery 38 is disconnected from its screen grid 35. To insure a positive choking action of the pentode amplifier stages I4 and I5 Y when no positive `screen grid bias is applied thereto, I provide batteries 39 and 40 connected with their negative terminal to the respective screen grids 34 and 35 through corresponding series resistances 1I and 42. `Accordingly, when the relay 30 is actuated to disconnect the positive bias 38 from a screen grid electrode, the negative bias of the batteries 39 and 40 are effective to suppress the passage of the carrier signals through the negatively biased amplifier stage. The function of resistances 4I and 42 is to limit the amount of current drained from the higher voltagepositive potential source 38 when it is connected to a screen grid electrode. The voltage of battery 38 is made sumcient to neutralize f the effect of negative bias batteries 39 and 4I) and provide normal positive biasing potential for the screen grids When connected thereto.

In accordance with my present invention, the signal selector mechanism 23 such as a printing telegraph distributor, actuates alternately the marking and spacing solenoids to correspondingly attract the relay tongue 3l for alternately contacting relay`contacts 32 and 33 to correspondingly alternately normally bias the screen grid electrodes 34 and 35. When either screen grid is positively biased by relay 30, the corresponding carrier frequency A or B is transmitted. Since the amplifier stages I4 and I5 block the passage of the carrier frequency oscillations therethrough when not connected to the battery 38, the particular carrier frequency transmitted over antenna 22 corresponds to the particular relay contact 32 or 33 connecting with relay tongue 3l. It will now be evident that only one carrier frequency can be transmitted at one time by my novel circuit arrangement and that the carrier waves are transmitted at maximum amplitude condition. The interval for the transmission of these carriers corresponds to the duration of the relay contacting periods which in turn are dependent upon the code combinations of signals to be transmitted.

Figure 2 is a diagram of a preferred receiving circuit for my present invention. vThe radiated signals Yfrom the transmitter are intercepted by antenna 50 and amplified by the selective sharply tuned radio frequency amplifier 5I. A crystal oscillator 52 is used to generate a currentof predetermined frequency for heterodyning with the received signals. The oscillator is connected to the heterodyne detector 53 for beating with the radio frequency signals to produce an audio frequency corresponding to the difference between the oscillator 52 frequency and the frequency of the received signals. The output of detector 53 is connected to a potentiometer 54 and in turn connected to the grids 55 and 55 of the dual amplifier stage 51. 'I'he output of the dual amplifier stage 51 is duplicated and similar at the points 58 and 59 from which they are introduced to individual sharply tuned filter networks 60 and 5I.

The filters 60 and 5I are designed to be sharply selective so as to definitely differentiate between the resultant two audio frequencies seventy cycles apart. My present invention is rendered commercially practical by the' utilization of crystal oscillators for the transmitter R. F. generators A and B and a crystal oscillator for the heterodyne frequency generator 52 which is predetermined in accordance with thev transmitting stage frequencies. Whenl a difference frequency of seventy cycles between the transmitted radio frequencies A and B is employed, the audio frequencies 60 and 6I accordingly must separate two audio frequencies which are also seventy cycles apart. The crystal oscillator 52 used for heterodyning with the received frequencies A and Bis designed to produce the predetermined audio frequencies for which the sharp lters 60 and 6I are designed to operate.

For example, if the A carrier frequency is 100,000 cycles and the-B carrier frequency is 100,070 cycles, the two carriers will be alternately radiated with a seventy cycle frequency difference'. Thel heterodyne crystal oscillator 52 may be designed to have a frequency of 99,500 cycles, producing at the output 54 of the heterodyne detector 53 audio signal frequencies of 500 cycles and 570 cycles. Accordingly, there will be impressed upon the grids 55 and 56 of the dual audio amplifier stage 51, alternate signal impulses of 500 and 570 cycles per second in .frequency. If the lter 60 is sharply tuned to pass 500 cycles and reject the 570 cycle signals, the other selector filter 5I Ais tuned to pass with maximum eiliciency the 570 cycle signals and reject the 500 cycle signals. Therefore, the output point 52 .of illter 50 will receive the 500 cycle signals which, in the present example, correspond to the marking impulses since they are produced by heterodyning with the A oscillator frequency. The output point 63 of filter 6I receives only the 570 cycle'impuls'es corresponding to the spacing impulses transmitted. The filters 50 and 6I may be designed according to principles Well known in the radio art and the illustrated embodiment is composed of inductances and condensers enclosed in a grounded shielded structure.

The respective outputs 62 and 33 from filters 6I) and 6I are amplified by the dual audio frequency amplifier stage 64 in a conventional manner. The outputs 55 and 66 of amplifier 64 correspond to the signals received at points 62 and 63 but in amplified form. The 500 and 570 cycle signals occur alternately corresponding to the marking and spacing impulses generated by the signal selector mechanism 2l and may be used to actuate a local relay to translate these signals in a manner well known to those skilled in the art. The marking and spacing signals are positively actuated since the amplitude of the respective signal impulses are equal. The alternating current signals may be used with corresponding alternating current relays. However, I prefer to rectify these individual and segregated signals before actuating the translator relays.

Accordingly, I employ detector stages 01 and Il, the inputs of which are connected respectively to the outputs 85 and 66 of the dual amplifier stage N. By-passing condensers C! and are connected across the anode-cathode circuits of the detector stages il and 88 to by-pass the alternating current components and permit the uni-directional signal impulses tobe impressed upon the differential relay li. A potentiometer .12 connected between ground and the B-terminal is used to supply a suitable bias to the grids of the rectiilerstages B1 and 08. The output of rectifier 61 is connected to terminal 'l2 of relay ll, and the output of rectifier stage 68 is connected to terminal 14 of relay 1|. The midpoint 'I5 of the relay 'lll is connected to the anode potential source B to complete the output circuits of the rectifier stages 61--88.

'Ihe following is an outline of the operation of the telegraphy system of my present invention. When the signal selector mechanism V23 actuates the marking solenoid 28, the relay vtongue 3| is attracted to connect with contact 32 to impress the positive operating bias potential upon the screen grid electrode Il in the amplifier train for oscillator A, relieving the blocking action of the amplifier stage Il and permitting the 100,000 cycle oscillator frequency to be transmitted over antenna' 22. At the receiver, the' heterodyne crystal oscillator of 99,500 cycles beats with the 100,000 cycle receivedvand amplified signals, to produce a 500 cycle audio frequency impulse at both the outputs 5B and 59 of the dual amplifier stage Il. The selective filter 60 which is sharply tuned to 500 cycles permits this signal to be further amplifled and impressed upon the rectifier stage 61. 'Ihe other filter 6| by-passes the 500 cycle signal to ground and prevents it from reaching the other rectifier 68. When the 500 cycles impulse reaches rectified stage 61, a uni-direc-V tional current ilows through the section I3-'l5 of the differential relay 'Hand attracts the tongue 'It of the relay to connect with contact 11 corresponding to the local marking impulse conditions.

The spacing signal impulse is transmitted in a manner similar to the marking impulse and corresponds to the attraction of spacing solenoid 29 of the relay tongue 3| away from-contact 32 to connect with contact 33. The oscillator A ampliiler train simultaneously blocks the transmission of the A carrier frequency and permits the B carrier frequency to pass through' the,pentode stage I5 to be transmitted over antenna 22. In

the present case, the B frequency is 100,070 cycles, which when heterodyned with the 99,500 cycle oscillator, produces the 570 cycle signal which passes through selector filter 6| to be rectified by rectifier 68. A corresponding uni-directional impulse passes through the coil section 'I5-'M of the differential relay to attract the relay tongue 1| to the spacing contact 18 for the local spacing circuit condition.

. The relay armature 'I0 together with the relay contacts 11 and 'I8 are connected to a local signal translator mechanism Il. The translator Il coranaal? responds to the type of selector mechanism 23 employed at the transmitter to suitably translate the transmitted signals. 'Ilia relay amature 'Il ispositivelyactuatedtothespacing andmarking positions.

It is to be understood that diiierent frequency ranges may be employed for the system of my present invention and that modincations therein which fall within the broader spirit and scope of my invention will be evident to those skilled in the art. Accordingly, I do not intend to be limited except as set forth in the following claims.

I claim:

- l. In a signalling system, means for producing alternate signal conditions to be transmitted: first and second oscillators arranged with inherent frequency stability to generate rst and second carrier currents having a predetermined low frequency difference; and means controlled by said signal conditions for transmitting said first carrier current during the occurrence of one signal condition and for transmitting said second carrier current during the occurrence of another signal condition comprising a ilrst bufl'er ampliiler stage having a grid electrode for amplifying said first current, and a second buffer amplifier stage having a grid electrode for amplifying said second current, means associated with said grid electrode for normally blocking the transmission of said currents, and means for permitting the transmission of said first or second currents in accordance with the signal conditions said normal blocking means being effective to rapidly block the transmission of signal energy from each amplifier at the termination of each operative period thereof whereby substantially continuous carrier wave signalling is eiiected.

2. In a signalling system, means for alternately producing signal conditions to be transmitted; first and second oscillators arranged with inherent frequency stability to generate rst and second radio frequency carrier currents having a predetermined low frequency difference; and means controlled by said signal conditions for transmitting said first carrier current during the occurrence of one signal condition and for transmitting said second carrier current during the occurrence of the other signal condition comprising' a first buffer amplifier stage having a grid electrode for amplifying said iirst current, and a second buier amplifier stage having a grid electrode for amplifyingsaid second current, means for normally blocking the transmission of said currents by negatively biasing said electrodes and means for applying normal biasing potential to said electrodes for permitting the transmission of said rst or second currents in accordance with the signal conditions, the negative biasing condition being eifective to rapidly block the transmission of signal energy from each amplifier when the normal biasing potential means is removed from the associated electrode, whereby substantially continuous carrier wave signalling is effected.

3. In a signalling system, means for alternately producing signal conditions to be transmitted; first and second oscillators arranged with `inherent frequency stability to generate rst and second radio frequency carrier currents having a predetermined low frequency difference; and means controlled by said' signal conditions for transmitting said first carrier current during the occurrence of one signal condition and for transmitting said second carrier current-during the occurrence of the other signal condition comprising a first buffer ampliiier stage having a screen grid electrode for amplifying said first current, and a second buffer amplifier stage having a screen grid electrode for amplifying said second current, means for normally blocking the transmission of said currents by negatively biasing said electrodes and means for applying normal biasing potential to Said electrodes and for counteracting said negative biasing condition for permitting the transmission of said first or second currents in accordance with the signal conditions, the negative biasing condition being eiective to rapidly block the transmission of signal energy from each amplifier when the normal biasing potential means is removed from the associated electrode, whereby substantially continuous carrier wave signalling is effected.

4. In a telegraph system, selector means for producing alternate marking and spacing signal conditions in accordance with character codes to be transmitted; first and second oscillators arranged with inherent frequency stability to generate rst and second radio frequency carrier currents having a predetermined low frequency difference; and means controlled by said signal conditions for transmitting said flrst carrier current during the occurrence of one signal condition and for transmitting said second carrier current during the occurrence of the other signal condition comprising a rst buffer ampliiier stage having a grid electrode for amplifying said first current, and a second buffer amplifier stage having a grid electrode for amplifying said second current, means for normally blocking the transmission of said currents by sufnciently negatively biasing said electrodes and means for alternately applying normal biasing potential to said electrodes for permitting the transmission of said first or second currents; said last mentioned means compris-g ing a relay having an armature and two contacts individually connected to said electrodes, a source of potential with the positive terminal connected producing alternate marking and spacing signal conditions in accordance with character codes to be transmitted; rst and second crystal controlled oscillators arranged with inherent frequency stability to generate first and second radio frequency carrier currents having a predetermined low frequency difference; and means controlled by said signal conditions for transmitting said rst carrier current during the occurrence of one signal condition and for transmitting said second carrier current during the occurrence of the other signal condition comprising a first buffer amplifier stage having a screen grid electrode for amplifying said first current, and a second buffer amplifier stage having a screen grid electrode for amplifying said second current, means for normally blocking the transmission of said currents by sufficiently negatively biasing said screen grid electrodes and means for alternately applying normal biasing potential to said screen grid electrodes and for counteracting said negative biasing condition for permitting the transmission of said first or second currents; said last mentioned means comprising a relay having an armature and two contacts individually connected to said electrodes, a source of potential with the positive terminal connected to said armature, said armature being controlled by said selector means for connecting said source of potential to the screen grid electrode of the amplifier of the current to be transmitted in response to the signal conditions whereby substantially continuous carrier wave` signalling is effected.

WILLIAM G. H. FINCH. 

